In adolescents being treated for ALL as well as in early survivors, this randomized study will therefore examine the effect of Vitamin D and calcium supplementation on correcting Vitamin D insufficiency and on improving bone density in the context of changes in body composition and body fat. Bone density will be measured by a radiology exam called a quantitative computed tomography (or "qCT") while body composition and body fat will be measured by a different radiology exam called a dual energy x-ray absorptiometry (or "DEXA" scan). The study will also examine in depth the relationship between these three elements - Vitamin D insufficiency, obesity, and ALL - and their impact on bone density.